Šimurková M., Ivan P. & Gargulák M., 2016: Subsolidus alkali metasomatism in the metamorphosed Ordovician acid volcanics and volcaniclastics of the Gelnica Group (Gemeric Superunit, Western Carpathians). Acta Geologica Slovaca, 8, 1, 87–98.

Subsolidus alkali metasomatism in the metamorphosed Ordovician acid volcanics and volcaniclastics of the Gelnica Group (Gemeric Superunit, Western Carpathians)

Mária Šimurková¹, Peter Ivan¹ & Milan Gargulák²

¹Department of Geochemistry, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynská dolina G, Ilkovičova 6, 842 15 Bratislava 4, Slovak Republic; simurkova@fns.uniba.sk, ivan@fns.uniba.sk
²D. Štúr State Geological Institute, Mlynská dolina 1, 817 04 Bratislava 11, Slovak Republic; milan.gargulak@geology.sk

Abstract

The Early Paleozoic Gelnica Group (Gemeric Superunit, Western Carpathians) is composed of low-grade metamorphosed flysch-like sedimentary complexes alternated with volcanogenic complexes built up mostly by acid volcaniclastic rocks. Volcaniclastic rocks and small rhyolite bodies Ordovician in age underwent subsolidus alkali metasomatism locally overprinted by multiple stages of younger metamorphic and hydrothermal alterations. The observed variability in chemical compositions indicates that the most of acid volcanic and volcaniclastic rocks experienced potassic or sodium metasomatic alteration, the unaltered or Mg-metasomatised rocks occur less frequently. The dominating K-metasomatism is related mainly to the K-feldspar, partly also biotite and muscovite/sericite formation at the expense of matrix or original feldspar phenocrysts. The Na-metasomatism is connected with albite formation. The highest concentrations of alkalies (up to 13.52 wt. % K₂O and 7.08 wt. % Na₂O respectively) have been found in rocks originally represented by glassy dacites. The elements like Al, Ti and Zr remain immobile in alkaline metasomatic processes in contrast to Rb or metals. Based on the results from areas with analogical geological structure and evolution, especially the Bergslagen area in Sweden, we suppose that alkali metasomatism in the Gelnica Group is a product of hydrothermal system caused by the infiltration and circulation of sea water. The areas with K-, Na- or Mg-metasomatism represent different parts of the original hydrothermal system, where the K-metasomatism was probably its lower temperature section. This hydrothermal system probably mobilised some metals (Cu, Pb, Zn, Fe, Mn) for the formation of stratiform sulfide and oxide mineralizations or served as the metal pre-concentrator for younger siderite-sulfide vein deposits.

Key words: Western Carpathians, Gelnica Group, Early Paleozoic, acid metavolcanites and metavolcaniclastics, alkali metasomatism

Manuscript received: 2016-02-01

Revised version accepted: 2016-06-08